Additive manufacturing (AM), also known as solid freeform fabrication or 3D printing, refers to a manufacturing process where three-dimensional objects are built up from raw material (generally powders, liquids, suspensions, or molten solids) in a series of two-dimensional layers or cross-sections. Machining techniques refer to subtractive processes and produce objects that are cut out of a stock material such as a block of wood or metal.
A variety of additive processes can be used in AM. The various processes differ in the way layers are deposited to create the finished objects and in the materials that are compatible for use in each process. Some methods melt or soften material to produce layers (e.g., selective laser melting (SLM) or direct metal laser sintering (DMLS), selective laser sintering (SLS), and fused deposition modeling (FDM)). Some methods cure liquid materials using different technologies, e.g. stereolithography (SLA).
Sintering is a process of fusing small grains (e.g., powders) to create objects from smaller grains using, for example, atomic diffusion. Sintering can involve heating a powder. The powder used in sintering need not to reach a liquid phase during the sintering process, in contrast to melting. When a powdered material is heated to a temperature below the melting point in a sintering process, the atoms in the powder particles diffuse across the boundaries of the particles such that the particles fuse together to form a solid piece. As the sintering temperature does not have to reach the melting point of the material, sintering can be used for materials with high melting points such as tungsten and molybdenum.
Both sintering and melting can be used in AM. The material being used determines which process occurs. An amorphous solid, such as acrylonitrile butadiene styrene (ABS), is actually a supercooled viscous liquid, and does not actually melt; as melting involves a phase transition from a solid to a liquid state. Thus, selective laser sintering (SLS) is the relevant process for ABS, while selective laser melting (SLM) is used for crystalline and semi-crystalline materials such as nylon and metals, which have a discrete melting/freezing temperature and undergo melting during the SLM process.